Trench cutters, also known as hydromills under their generic name, are used in foundation engineering processes, for instance to build diaphragm walls. Trench cutter systems comprise generally a frame, such as a steel frame, that can be lowered into the ground that is being excavated. Normally a motor and two cutter wheel pairs, which are arranged to be rotated by the engine, are installed at the end of the frame that is to be first lowered in the ground. Each wheel pair has a first wheel and a second wheel arranged so that one engine is arranged inside these two wheels or drums. The cutter can be lowered vertically under continuous rotation of the cutter wheels, and depths of more than 150 m can be reached. The advance is brought about by the weight of the cutter wheels and the frame, which is hung by means of cables to a crane. Due to the rotation of the cutter wheels, the ground below the wheels is continually loosened or broken down and conveyed back to the surface by using a mud pump just above the cutter wheels and a suction means between the wheels.
In the known solutions the hydromill wheels are normally equipped with different kinds of teeth or drag bits that are designed to come in contact with the ground and to perform the actual breaking of the ground. However, when excavating hard or very hard rock, these teeth or drag bits become inefficient, and the hydromill can be brought to a standstill, i.e. it is no longer able to penetrate the rock. In order to be able to continue the excavation, the hydromill has to be withdrawn from the trench, and a heavy chisel (typically 12 to 20 tons) has to be dropped several times on the rock to sufficiently fracture it first, before the hydromill can be brought again and resume excavation. As the fracturing effect of chiselling is limited in depth below the rock surface on which the chisel is dropped, this process of alternating chiselling and excavation with the hydromill equipped with drag bits or other types of teeth must be repeated several times, resulting in very slow progress. In addition, on some building sites with neighbouring sensitive structures, such as for example old buildings in poor condition, historical monuments or data centres, there are limits for the allowed vibrations, and consequently chiselling is sometimes prohibited on these sites.
To mitigate this problem it has been designed to use rollers with button bits (rounded studs) instead of the teeth or drag bits. However, this solution is also not optimal, since cutter systems have a limited weight, and in the case of rollers with button bits, there would be too many button bits in contact with the rock at the same time, resulting in an insufficient pressure to crush the rock at a particular time.
It is the object of the present invention to overcome the problems identified above related to excavating hard or very hard material, such as rock.